Can a Spiked Gauntlet worn by a polearm-wielding Fighter allow him to do attacks of opportunity with it?

If a fighter is wearing Spiked Gauntlets wielding a Ranseur, does he threaten both the 5′ area around him with the piked gauntlets and the 10′ with the Ranseur which would allow him to make an attack of opportunity with either weapon depending on the distance of his opponent?

Can a character with the Polearm Master feat make an opportunity attack against an invisible creature that enters their reach?

The second benefit of the Polearm Master feat (PHB, p. 168) says:

While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach.

The regular rules on opportunity attacks state:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

Obviously, Polearm Master lets you use your reaction to make an opportunity attack when a creature enters your reach, in addition to the opportunity attack any character can make when a creature leaves their reach. However, does the opportunity attack from Polearm Master follow the same restriction on needing to see the creature, or can this attack be made even without seeing the creature that enters its reach?

If an invisible creature enters the reach of a character with the Polearm Master feat, can the character make an opportunity attack against that creature?

Can an ankheg make an opportunity attack while grappling?

The Ankheg has one single attack action (aside from its acid spray):

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage plus 3 (1d6) acid damage. If the target is a Large or smaller creature, it is grappled (escape DC 13). Until this grapple ends, the ankheg can bite only the grappled creature and has advantage on attack rolls to do so.

So if it attacks someone with a bite and hits, it grapples them and can’t bite anyone else.

What happens if someone else moves out of its attack range? Can it not make an opportunity attack because it can’t use its Bite ability? Or does it make an “unarmed” attack?

Does attacking with a reach weapon provoke an Attack of Opportunity?

There’s plenty of scholarship on making attacks of opportunity with a reach weapon; that’s not what I’m asking about. I’m asking about, if you’re standing adjacent to an enemy with a normal-range weapon, and you are making an attack on some third party with a reach weapon, does that provoke an AoO from the adjacent enemy?

The table of “Actions in combat” specifies that a melee attack provokes an AoO and a ranged attack does not. A reach attack is a melee attack, but it seems absurd that you could attack far away with a pike or longspear and still defend yourself—it clearly ought to provoke an AoO like a ranged or magical attack does. But the rules as written don’t seem to actually cover it at all.

Can a character with the War Caster feat call a bolt with Call Lightning instead of making an opportunity attack?

One of my players has a 6th level druid character, and he asked me if, as long as he had the War Caster feat, he could invoke an active Call Lightning spell and use a reaction to call down a new bolt of lightning as an opportunity attack.

Is it possible to use War Caster and Call Lightning this way?

Reach and Attacks of Opportunity: How to treat an approaching enemy?

Imagine this scenario:

Paint skills

Red has a reach weapon and he’s threatening B1 and B2.

Green has a normal weapon and wants to get closer to Red in order to attack him. ( C2 )

So green takes a first step into the reach of Red’s weapon, but provokes no attack of opportunity (not yet).

Green then takes a second step into C2, leaves the threatened area of red and arrives in C2.

This should normally provoke an attack of opportunity, but since Red is not threatening C1 and C2, I’m not sure if he gets the AoO.

Green is using no particular feat or acrobatics check to avoid the AoO.

Actually, I found no evidence in rules as written, so any reference is gladly welcome.

In 5e, can you draw and or throw a javelin on one or more successive turns while still Using a 2 handed weapon for opportunity attacks?

Assuming you do not have any extra attacks involved, can you start your turn holding a 2 handed weapon, use your free action to draw a javelin, then throw the javelin with your attack and return your hand to your 2 handed weapon for opportunity attacks (such as granted by pole arm master)?

Does an opportunity attack give everyone (potentially) a 2nd or even 3rd attack (as a reaction) per round?

Opportunity attacks have me confused; I’m wondering why any creature would flee in a battle knowing that it could be potentially attacked by several combatants as a reaction. Along the same same lines, if a pc has taken action and reaction, does this mean that they are no longer able to make an opportunity attack in that round of combat?

How does this change to the opportunity attack rule impact combat?

My DM has introduced a house rule affecting opportunity attacks, and I am trying to understand the implications of this change.

The new rule is this:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach or moves inside your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.

I’ve bolded the portion added by the house-rule.

To clarify, when a creature moves from out of a character’s reach into their reach, it does not provoke an opportunity attack. But if a creature moves within a character’s reach (i.e. from one spot within reach to another), it provokes an opportunity attack.

I think this can reduce the utility of the Rogue (with Sneak Attack), and kind of reduce the Mobile feat’s benefits. Are there other places where this rule can change how combat is played? Like, what can become imbalanced with this ruling?

My goal is to better understand this mechanic before talking with my DM.

A foe leaves the reach of my 5-foot reach sword. Can I make an Opportunity Attack with my 10-foot reach whip?

This is a follow up to the question: When are Opportunity Attacks provoked while holding a reach and a non-reach weapon?

The answers establish pretty solidly that if I hold a weapon with a reach of 5 feet (say, a sword) and another weapon with a reach of 10 feet (say, a whip), an adjacent foe still triggers an Opportunity Attack if they move 5 feet away from me.

However, it is not clear to me if I can use either weapon for my Opportunity Attack, or if I have to use the weapon the enemy moved out of the reach of.

Can I use either weapon for the Opportunity Attack or does it have to be the weapon that the enemy leaves the reach of?